There is no such thing as a bad incoming link. The worst that can a link can do is nothing at all. It only gets better from there. If this wasn't true your competitors could ruin you by linking to you with crappy sites.

And it should be very easy to tell if a link is good or bad. Look at the site/page it is on. Is the site any good? Or does it suck? Does it rank well? Or not? It really is easy to tell.